Flexible current sensors are based on flexible inductive coil which can be formed to create a closed path around the measured current carrying conductor. The voltage induced into the coil is proportional to the derivation of the total current flowing through the enclosed area. This principle of current measurement is well known as the Rogowski principle or the Rogowski coil following the publication by Rogowski W. and Steinhaus W. in 1912. A flexible current sensor generally has a form of sensing cable equipped with a mechanical coupling system for either fixing or releasing the sensor cable ends to and from mutually a closed position. The sensing cable consists of sensing coil wound on cylindrical flexible core, optional electrical shielding located over said sensing coil and outer isolation coat. In order to convert the coil voltage to output signal proportional to current an electronic integrator circuit is usually part of the flexible current sensor. The flexible sensors are advantageous for measurement of current in large size wires, difficult shape conductors and conductor groups.
Known constructions of the flexible current sensors have lower amplitude and phase accuracy compared to ferromagnetic current transformer types. The accuracy of flexible current sensors is affected mainly by sensor coil inhomogenity and by residual signals captured via capacitive and inductive coupling to other voltage and current sources. Due to low level of the output voltage of the sensor coil they are sensitive to disturbance, interference and noise generated in evaluation electronics. Commercially available flexible current sensors on Rogowski principle are due to insufficient accuracy dedicated mainly for informative measurements.
An attempt to arrange the classic ferromagnetic current transformer based on soft magnetic material core into flexible probe is disclosed in international publication WO96/28737 (Upton) comprising a helical winding along flexible support comprising an elongate strip of soft magnetic material such as a matrix medium in which particles of low magnetic impedance material are embedded. Disadvantage of this approach is the nonlinearity inherent to any soft magnetic material. This nonlinearity substantially affects the accuracy of the current transformer. Another disadvantage of the disclaimed composite soft magnetic material core are inherent non-magnetic gaps between magnetic particles lowering the effective permeability and thus leading to low winding inductances and mutual coupling resulting to significant phase and amplitude errors.
Relevant solutions for Rogowski coil arrangements and respective electronics can be found in following references:
U.S. Pat. No. 4,689,546 (Stephens et al) discloses arrangement of plurality of classic single-coil flexible Rogowski sensors with conventional metallic shield (FIG. 10). The individual sensors are differentially arranged using differential amplifier to sense error current of the power generator. This arrangement in form of air-core current transformers disposed over conductors of three-phase generator serves as current monitoring equipment for generator control system.
United Kingdom Patent No. GB2259150A discloses arrangements of electronic integrator with combined feedback.
U.S. Pat. No. 5,442,280 discloses fixed Rogowski coil arrangements on printed circuit layout.
European Patent No. EP652441A1 discloses fixed Rogowski coil arrangements for installation with earthed metal casing.
U.S. Pat. No. 6,614,218B1 discloses arrangements of combined passive and active integrator for Rogowski coil. The presented combined integrator shows higher frequency band compare to classic simple integrator but at the expense of lower overall accuracy.
European Patent No. EP0834745A2 discloses arrangement of fixed Rogowski coil with high homogeneity.
Japanese Patent No. JP2000065866 discloses multiple Rogowski coil arrangement for measurement of current flowing through large structure.
United Kingdom Patent Application No. GB2332784A discloses flexible Rogowski coil arrangements consisting of sensor coil wrapped on fixed support structure.
German Patent Application No. DE19811366A1 discloses flexible Rogowski coil arrangements optimized for repeated assembling on power line.
United Kingdom Patent No. GB2342783A discloses fixed Rogowski coil arrangements on printed circuit layout.
German Patent Application No. DE19959789A1 discloses arrangements of electronic integrator with switched DC feedback.
U.S. Pat. No. 6,825,650B1 discloses arrangements of series of fixed Rogowski coils for current measurement in electricity meter.
U.S. Pat. No. 6,313,623B1 discloses spatial arrangements of two Rogowski coils eliminating residual signals.
United States Patent Application Publication No. US2003/0090356A1 discloses fixed Rogowski coil arrangements on printed circuit layout with optic transmission path.
U.S. Pat. No. 6,963,195B1 (Berkcan) discloses fixed Rogowski coil arrangement equipped with additional electrostatic shielding coil. Disadvantage of using single coil as electrostatic shielding is its high impedance resulting to weak shielding effect for strange AC field.
Patent abstract of Japan Publication No. 02118460 (Energy Support Corp) discloses various arrangements of known classic Rogowski coil optimized for measurement of partial current which penetrates a cylinder body.
U.S. Pat. No. 5,012,218 discloses an arrangement of fixed solenoid using Rogowski coil principle for measurement of DC current. The device is equipped with gap for easy positioning of the current carrying conductor into and outside the sensing area of the coil. The influence of the gap is minimized by compensation coils located at the gap ends.
German Patent No. DE2432919A1 (Siemens AG) discloses continuous method of winding of multiple coils for current transducer consisting fixed rigid coils arranged around current caring conductors.
The basic principle of making fixed coil from metal tracks creating pair of go and return winding on the PCB is disclosed in EP0573350 (Dupraz et al).
United States Patent Application Publication No. 2005/248430A1 (Dupraz et al) discloses similar arrangement of fixed Rogowski sensor coil manufactured via PCB technique assembled from partial individually manufactured segments. Similar approach is disclosed in German Patent No. DE10161370 or United States Patent Application Publication No. US2003/137388A1 where the fixed Rogowski coil is constituted by semi-annular portions fabricated via printed circuit technique. The portions are arranged in stack and electrically interconnected to approach the homogeneous coil.
Rogowski coil is can be advantageously used for extremely high amplitude and rise rate current measurement like lightning. Arrangement of fixed Rogowski coil for this purpose is described by Torre et al “Design, construction and calibration of three devices to measure directly lightning parameters”. The self-integrating Rogowski coils is built with resistive wire wound on acrylic core. This arrangement acts for high frequencies as integrating device not needing additional integrator.
The extreme linearity, lack of saturation of the non magnetic core and practically non limited frequency range are inherent property of the Rogowski principle based current sensors. These properties still invoke demand for their improvement and application oriented optimization.
Therefore, it is an object for this invention to provide new and improved arrangement of universal generally applicable flexible current sensor and its evaluation electronics yielding higher accuracy comparable to fixed coil arrangements and improvements in its applications.